1. Field of the Invention
The present invention relates to a tunnel magnetoresistive element utilizing the tunnel magnetoresistive effect, a thin-film magnetic head and a memory element incorporating the tunnel magnetoresistive element, and methods of manufacturing such a tunnel magnetoresistive element, thin-film magnetic head and memory element.
2. Description of the Related Art
Performance improvements in thin-film magnetic heads have been sought as recording density of hard disk drives has increased. Such thin-film magnetic heads include composite thin-film magnetic heads that have been widely used. A composite head is made of a layered structure including a recording head having an induction-type electromagnetic transducer for writing and a reproducing head having a magnetoresistive element for reading.
Reproducing heads that exhibit high sensitivity and produce high outputs have been required. In response to such demands, attention has been focused on tunnel magnetoresistive elements (that may be hereinafter called TMR elements) that detect a magnetic field through the use of the tunnel magnetoresistive effect.
The TMR element has a structure in which a lower magnetic layer, a tunnel barrier layer and an upper magnetic layer are stacked. Each of the lower and upper magnetic layers include a ferromagnetic substance. In general, the magnetic layer closer to the substrate is called the lower magnetic layer and the magnetic layer farther from the substrate is called the upper magnetic layer. Therefore, the terms ‘upper’ and ‘lower’ of the upper and lower magnetic layers do not always correspond to the position in the arrangement of an actual TMR element.
The tunnel barrier layer is a layer made of a thin nonmagnetic insulating film through which electrons are capable of passing while maintaining spins thereof by means of the tunnel effect, that is, through which a tunnel current is allowed to pass. The tunnel magnetoresistive effect is a phenomenon that, when a current is fed to a pair of magnetic layers sandwiching the tunnel barrier layer, a tunnel current passing through the tunnel barrier layer changes, depending on the relative angle between magnetizations of the two magnetic layers. If the relative angle between magnetizations of the magnetic layers is small, the tunneling rate is high. As a result, the resistance to the current passing across the magnetic layers is reduced. If the relative angle between magnetizations of the magnetic layers is large, the tunneling rate is low. The resistance to the current passing across the magnetic layers is therefore increased.
As the recording density is increased, a magnetic recording pattern stored on a recording medium is reduced. It is required to reduce the size of a TMR element incorporated in the reproducing head, accordingly. For example, to meet recording density of 40 gigabits per square inch, the size of the TMR element is required to be as small as 0.4 μm by 0.4 μm.
Reference is now made to FIG. 36 to FIG. 39 to describe an example of a method of making a small-size TMR element as described above. In this method, as shown in FIG. 36, a lower electrode layer 101 and a film 112 to be the TMR element are formed one by one on a substrate not shown. Next, a fine resist mask 113 using a positive resist is formed by photolithography on the film 112. Next, as shown in FIG. 37, the film 112 is selectively etched through ion milling, for example, using the resist mask 113, to form the TMR element 102. Next, as shown in FIG. 38, an insulating layer 103 is formed around the TMR element 102 to insulate the lower electrode layer 101 from an upper electrode layer described later. Next, as shown in FIG. 39, the resist mask 113 is removed and the upper electrode layer 104 is formed on the TMR element 102.
When the TMR element is formed by etching the film to be the TMR element through ion milling, for example, as described above, a substance separated by etching deposits on sidewalls of the TMR element and a deposition layer is thus formed. As a result, this deposition layer causes a reduction in the properties of the TMR element. Such a problem is caused by a phenomenon called extra current channels, that is, the deposition layer forms channels of extra currents that do not contribute to the tunnel magnetoresistive effect. (See Olivier Redon et al., '44th Annual Conference on Magnetism and Magnetic Materials' AA-11, 1999.) To prevent such a reduction in the TMR element properties, it is desired to avoid formation of a deposition layer if possible when the film to be the TMR element is etched. However, it is impossible in practice that no deposition layer would be formed.
In Published Unexamined Japanese Patent Application Heisei 11-96512 (1999), a technique is disclosed for preventing rust resulting from a deposition layer of a magnetic material formed in the step of ion milling. According to this technique, an insulating protection layer is formed to cover the deposition layer, after etching is completed, in the chamber in which etching has been performed. However, although this technique is utilized for making a TMR element, extra current channels are still formed by the deposition layer. It is therefore impossible to solve the above-mentioned problem.